The vertical line space between the lines of type. The term originated in the days of metal type. During hand-typesetting, thin strips of lead were inserted into the lines of type to increase the distance.
•Leading is the distance, in points, from the baseline of one character to the baseline of the line of text above
•So if you see 12/24, that means 12 pt of type with 24 pts leading

https://blog.shillingtoneducation.com/leading-typography/

https://www.skillshare.com/en/blog/what-is-leading-in-typography/

https://fonts.google.com/knowledge/glossary/line_height_leading

Ellen Lupton Book Thinking with type